Fixed-wing aircraft use wings to generate lift in response to the forward airspeed of the aircraft. Forward airspeed is generally maintained by forward thrust from one or more jet engines or propellers. For the wings to generate sufficient lift, a fixed-wing aircraft must achieve takeoff speed and therefore requires a runway for takeoff and landing.
Unlike fixed-wing aircraft, vertical takeoff and landing (VTOL) aircraft do not require runways. Instead, VTOL aircraft are capable of taking off, hovering and landing vertically. One example of VTOL aircraft is a helicopter, which is a rotorcraft having one or more rotors that provide vertical lift and forward thrust to the aircraft. Helicopter rotors not only enable hovering, vertical takeoff and vertical landing, but also enable, forward, aftward and lateral flight. These attributes make helicopters highly versatile for use in congested, isolated or remote areas where fixed-wing aircraft may be unable to takeoff and land. Helicopters, however, typically lack the ability to achieve high forward airspeed or the forward flight efficiency of fixed-wing aircraft.
A tiltrotor aircraft is another example of a VTOL aircraft. Tiltrotor aircraft utilize tiltable rotor systems that are operable to transition between a vertical lift orientation and a forward thrust orientation. The rotor systems are tiltable relative to a fuselage and/or a fixed wing such that the associated proprotors have a generally horizontal plane of rotation for vertical takeoff, hovering and vertical landing and a generally vertical plane of rotation for forward flight, wherein the wing provides lift. Taking advantage of these features, tiltrotor aircraft combine the vertical lift capability of a helicopter with the speed and range of fixed-wing aircraft. It has been found, however, that the large diameter rotor systems of tiltrotor aircraft can become unstable as speed increases.